The present invention relates to light emitting devices. More particularly, the present invention relates to light emitting device modules and lighting devices.
Light emitting diodes (LEDs) are typically made using semiconducting material doped with impurities to create a P-N junction. When electrical potential (voltage) is applied to the P-N junction current flows through the junction. Charge-carriers (electrons and holes) flow in the junction. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of light (photon, radiant energy) and heat (phonon, thermal energy).
In most applications, light is the desired form of energy from an LED and heat is not desired. This is because heat can and often causes permanently damages to the LED, degrades LED performance by causing decreased light output, and leads to a premature device failure.
However, in the current state of art, generation of undesired heat cannot be avoided. A typical high power LED chip of 1 mm2 in area and 0.10 mm in thickness has a P-N junction active layer of only 0.003 mm thick. Yet, it can convert 1 to 2 watts of electrical energy into both radiant and thermal energy. More than 50% of electrical energy is actually converted into thermal energy which can heat up the whole LED within fraction of a second. Typically, such LED operates at a junction temperature of 120 degrees Celsius. That is, these LEDs operate at a temperature greater than the temperature of boiling water (water boils at 100° C.). Above 120 degrees C., the LED's forward voltage will increase, thus resulting in higher power consumption. Also, its luminous output will drop correspondingly and its reliability and life expectancy will also be adversely affected.
The problem of heat is even more apparent for high power LEDs. There is an increasing demand for increasingly brighter LEDs. To make brighter LEDs, the most obvious solution is to increase the electrical power applied to the LEDs. This however leads to LEDs operating at even greater temperatures. As the operating temperature increases, the efficiency of the LEDs decreases, resulting in light output that is less than expected or desired. That is, for example only, doubling the electrical power of the LED does not result in the generation of twice the amount of light. Rather, the light output is much less than the expected twice the luminosity.
The problem of heat is compounded by the way in which the LEDs are packaged within light emitting devices such as light bulbs. Light emitting devices of current art (using LEDs as the core of the device) often entrap heat within the device itself. This decreases the expected life of the LED and of the device itself. For example, many LEDs in the marketplace are sold as having expected operating life of 50,000 hours (at which time the LED output declines to seventy percent of its original output). However, light emitting devices (having such LEDs as the light emitting element of the device) typically specifies only 35,000 hours of expected operating life).
Accordingly, there remains a need for an improved LED module that eliminates or alleviates these problems associated with heat.